1. Field of the Invention
The present invention relates to speaker enclosures and particularly to ducted port speaker enclosures.
2. Description of Prior Art
Sound may be defined as vibrations produced by an object. Whenever an object vibrates, it causes the surrounding air to compress and expand and, in turn, the compressions and expansions of the air near the source of the sound cause compressions and expansions in the air further away. The compressions and expansions are called sound waves. The pitch of the sound is determined by the frequency of its vibrations and is measured in hertz or Hz. The intensity of the sound is determined by the amplitude or distance the vibrating object moves as it vibrates and is measured in decibels or dB. Pitch affects the loudness but not the intensity of a sound where loudness is the apparent strength of the sensation received by the brain and the intensity refers to the amount of energy in the sound waves. The ear has a low sensitivity to low pitches such as the lowest note on a piano having a frequency of 27 Hz and some organ notes as low as 15 Hz. A sound with an intensity of 60 dB sounds louder at a frequency of 1,000 Hz than at 500 Hz so that a greater intensity is required at the lower frequencies to produce a loudness equivalent to that at the higher frequencies. The low frequencies from such instruments as the tympani and the bass guitar may be lost by the average listener because the intensities of the low frequencies either from recordings or even live performances may be below the range at which these vibrations may be heard.
Phonograph recordings are made with the use of microphones which change the sound waves into electric currents while the loudspeaker changes the electric currents back into sound waves. The loudspeaker must not only reproduce the sounds from the electric currents but must also provide means for in some way increasing the intensity of low frequency sounds to which the ear has relatively low sensitivity. The low intensities may be caused by losses occurring because of the sound reproducing process including the recording studio, the record processing, the turntable, and the speakers as well as the associated amplifiers, wiring, connectors, and the like.
Sound waves will expand outwardly from the source of the sound similar to ripples in a pool of water which are created when a pebble is dropped into it. When a wave encounters an interference such as a wall which forms a reflecting surface, the wave will be reflected and again expand but in a new direction depending on the orientation of the reflecting surface. In a speaker cabinet the sound wave will be reflected by each of the walls it encounters and the reflected waves will also encounter the walls to be again reflected while new waves are created by the vibrations of the speaker diaphragm. Each time there are new vibrations the process is repeated until the various elements of the composite sound in effect become turbulent, set up standing waves, resonance effects, and other phenomena which are exceedingly difficult to analyze and predict. Consequently, although the net effect may be capable of being measured, the causes can usually only be hypothesized because of the difficulty of examining the complexity of the sound waves within the cabinet under the influence of the vibrating diaphragms. Various formulae and concepts have been suggested to predict the performance of speaker enclosures, however, in the final analysis, the human ear is the determining factor as to whether a particular speaker enclosure is producing sounds at the proper intensities over the listening range.
Speaker enclosures for enhancing the low frequency sounds are well known in the art and are usually referred to as bass-reflex or ported boxes; the ports which are a combination of a sound duct and its associated port opening are sometimes referred to as ducted ports. They contain one or more woofers or low-frequency range speakers, tweeters or high-frequency speakers, and probably one or more mid-range speakers, and are tuned with ducted ports. As described by David Weems in "Building Speaker Enclosures", published by TAB Books, Inc., Catalogue No. 62-2309 of Radio Shack, the design and construction of several types of ported boxes are developed from mathematical formulae and curves. For example, the speaker's bass performance in ported boxes of various cubic dimensions include these specifications: f.sub.s --the driver's frequency of free air resonance; Q.sub.TS --the driver's resonance magnification at resonance; and V.sub.AS --the driver's compliance stated in terms of the air volume that has an equivalent compliance for that driver. From these specifications, a determination is made of box volume, box resonance frequency and system cut-off frequency using charts, pocket calculator or computer. For the Thiele ported box, for example, for f.sub.s =25 Hz, the box volume of 15 cubic feet is required; for f.sub.s =40 Hz, a volume of 5 cubic feet is required. A lower frequency is achieved by employing the ported ducts. U.S. Pat. No. 3,037,081 discloses an acoustic resonator with a woofer and a ducted port wherein there is a combination of a low value of the volume parameter and various combinations specified in order to increase the equivalent mechanical resistance in series with the diaphragm of the loudspeaker. U.S. Pat. No. 3,952,159 discloses a bass-reflex reproducing system employing a woofer, a tweeter, and a ducted port and exhibiting a predetermined compliance in order to achieve the low frequency response.
There is a need for a small inexpensive speaker enclosure which can be readily constructed by a do-it-yourselfer which is based on experimental determinations of the various enclosures and ducted port parameters to achieve the desired low-frequency range of sound rather than based on computations which do not factor the human response into its output. The equations used in the prior art described hereinbefore are based generally on broad assumptions as to the behavior of the sound waves within the enclosure and are approximations as to the true character of the predicted sound output. Usually complex tuning adjustments are required to achieve pleasing results over the broad band of reproduced sound when designs are based solely on computations in other than relatively large and expensive enclosures.